This invention relates to glass to metal seals of the type used for obtaining hermetic seals in microcircuit cases and the like. A microcircuit case typically has a number of feed through connector pins positioned in glass insulators in one or more of the case walls for making electrical connections between the microcircuit within the case and the adjacent circuitry. The usual construction consists of a metal lead serving as an electrical conductor, with the metal lead mounted in an opening in the metal wall with a glass insulating sleeve.
Two forms of seals are used for providing electrical connections through a metal wall. One type is called a matched seal, wherein both the conductor and case are formed of a material which has a coefficient of thermal expansion substantially matching the coefficient of thermal expansion of the insulating material, typically a glass. In this type of construction, the glass and the conductor change shape with temperature in substantially the same amount so that the seal between the glass and case and the glass and conductor is not adversely affected by thermal cycling. Typically the metal used for the conductor and case is a special iron-nickel-cobalt alloy (29% nickel, 17% cobalt, 53% iron and 1% minor ingredients) known as ASTM-F15.
A second type of seal is called a compression seal, wherein the coefficient of thermal expansion of the glass insulator is greater than that of the conductor, but less than that of the case. Then with changes of temperature within the expected range of operation, the sealed surfaces remain pressed firmly together at all times so that a hermetic seal is maintained. A variety of materials may be used for the conductor and case if the proper order of the coefficient of thermal expansion for the materials is maintained. Typical conductor materials are ASTM-F15 or alloy-52, and typical case materials are cold rolled or stainless steel.
Both matched seals and compression seals have been used in the past, and both have been satisfactory for many situations. However the increased demand for more reliable hermetic seals with increased thermal cycling requirements and at the same time with lower costs, has made the present designs unsatisfactory for some applications.
A typical prior art case device is shown in U.S. Pat. No. 3,548,076. The electrically conducting leads are positioned in a metal wall by glass seals. U.S. Pat. No. 3,370,874 shows the use of an iron oxide layer on the exterior of the lead for improved sealing with the glass. The iron oxide layer has proved satisfactory in some situations, but problems are encountered in other situations. The iron oxide layer typically is produced in a furnace. To maintain the proper atmosphere and temperature stability, large furnaces are normally used even for oxidation of relatively small components. The iron oxide layer has become a less cost effective method of manufacture as the relative cost of energy has increased. A second problem with the iron oxide layer is that it tends to cause the glass to climb up the sealed lead. As the demand for small microcircuit cases with a large number of small leads increases, this tendency for the glass to climb up the lead results in a cone shape of the glass surface extending away from the case. The cone shaped glass surface is prone to handling damage.
It is an object of the present invention to provide a new and improved glass to metal seal for mounting an electrical conductor in a metal wall, and a method of sealing a metal lead in a metal wall, which provides improved hermetic sealing, particularly with thermal cycling, and at the same time is relatively simple and inexpensive to produce.
Other objects, advantages, feature and results will more fully appear in the course of the following description.